8.5 Thermodynamic Information from Electrochemistry 379
operating reversibly between 1000◦C and 100◦Cif
2.000 mol of hydrogen gas and 1.000 mol
of oxygen gas are reacted. Assume that the water
is produced as vapor, that the enthalpy change is
approximately equal to that at 298.15 K, and that all
of the enthalpy change is transferred to the engine.
Compare your answer with the result of part a.
c.Calculate the minimum amounts of wasted energy
in the form of heat for parts a and b of this problem.8.39A fuel cell with an acidic electrolyte using methane as a
fuel has the anode half-reaction equation
CH 4 (g)+2H 2 O(l)−→CO 2 (g)+8H++ 8 e−a.Write the cathode half-reaction equation and the cell
reaction equation for the methane fuel cell. Calculate
∆G◦andE◦for the cell reaction.
b.Fuel cells have a thermodynamic advantage over
energy delivery systems in which fuel is burned in an
internal combustion engine or a turbine in order to drive
an electric generator. Calculate the maximum amount
of work that can be obtained from the combustion of
100.0 kg of methane in a turbine or other heat-driven
engine with an upper temperature
of 2000 K and an exhaust temperature of 1200 K.
c.Calculate the maximum energy in kilowatt-hours
that can be obtained from the work in part a, assuming
that one has an electric generator that is 100% efficient.
d.Calculate the maximum energy in kilowatt-hours that
can be obtained from a fuel cell that consumes 100.0 kg
of methane. Assume that all products and reactants are
in their standard states. Comment on the advisability of
using fuel cells for municipal electric power generation.
What disadvantages might such use have?8.40 Find the activity of chloride ion necessary so that chlorine
instead of oxygen will be evolved when an infinitesimal
current flows at 298.15 K in an electrolysis cell containing
an aqueous solution of HCl.
8.41 Identify the following statements as either true or false. If a
statement requires some special circumstance to make it
true, label it as false.
a.Any electrochemical cell can be used to carry out a
chemical reaction reversibly.
b.When an electrochemical cell is changed from a
galvanic cell to an electrolytic cell by increasing the
counter e.m.f. imposed by the external circuit, the
electrode that was the anode now becomes the cathode.
c.A salt bridge minimized a liquid junction potential by
creating two liquid junctions whose liquid junction
potentials partially cancel.
d.The Nernst equation is an approximate equation whose
validity must be verified for each case.
e.Activity coefficients for individual ions cannot be
measured directly.
f.Electrochemical data can be used to obtain values of
equilibrium constants only for oxidation-reduction
reactions.
g.Fuel cells are limited in their efficiency by the Carnot
efficiency formula.
8.42A student at the University of Denver measured the pH
of a solution using a hydrogen electrode and a calomel
reference electrode at 25◦C and a barometric pressure
of 630 torr. When she calculated the pH, the result was
that pH 3 .00, but she forgot to correct for the
fact that the hydrogen gas was not at its standard state.
If the barometric pressure was 630 torr, what pH
should the student have reported?